A sintered R-T-B based magnet, including an R2T14B type compound as its main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a motor for a hybrid car and in numerous types of consumer electronic appliances. As a sintered R-T-B based magnet loses its coercivity at high temperatures, such a magnet will cause an irreversible flux loss. For that reason, when used in a motor, for example, the magnet should maintain coercivity that is high enough even at elevated temperatures to minimize the irreversible flux loss.
It is known that if R in the R2T14B type compound phase is partially replaced with a heavy rare-earth element RH, the coercivity of a sintered R-T-B based magnet will increase. It is effective to add a lot of such a heavy rare-earth element RH to the sintered R-T-B based magnet to achieve high coercivity at a high temperature. However, if the light rare-earth element RL is replaced with the heavy rare-earth element RH as R in a sintered R-T-B based magnet, the coercivity (which will be referred to herein as “HcJ”) certainly increases but the remanence (which will be referred to herein as “Br”) decreases instead, which is a problem. Furthermore, as the heavy rare-earth element RH is one of rare natural resources, its use should be cut down.
For these reasons, various methods for increasing HcJ of a sintered magnet effectively without decreasing Br by adding as small an amount of the heavy rare-earth element RH as possible have recently been researched and developed.
Patent Document No. 1 discloses a method for producing a sintered R-T-B based magnet which includes the steps of: loading a sintered R-T-B based magnet body and an RH diffusion source including a metal or alloy of a heavy rare-earth element RH into a processing chamber so that the magnet body and the diffusion source are movable relative to each other and readily brought close to, or in contact with, each other; and performing an RH diffusion process by conducting a heat treatment on the sintered R-T-B based magnet body and the RH diffusion source at a temperature of 500° C. to 850° C. for at least 10 minutes while moving the magnet body and the diffusion source either continuously or discontinuously in the processing chamber. Such a method contributes to increasing HcJ without decreasing Br by diffusing a heavy rare-earth element RH such as Dy or Tb inward from the surface of a magnet material.
Patent Document No. 2 discloses a method for diffusing a heavy rare-earth element RH such as Dy inside from the surface of a sintered magnet body of an R-T-B based alloy while supplying the heavy rare-earth element RH onto the surface of the sintered magnet body (which will be referred to herein as an “evaporation diffusion process”). According to Patent Document No. 2, inside of a processing chamber made of a refractory metal material, the sintered R-T-B based magnet body and an RH bulk body are arranged so as to face each other with a predetermined gap left between them. The processing chamber includes a member for holding multiple sintered R-T-B based magnet bodies and a member for holding the RH bulk body. A method that uses such an apparatus requires a series of process steps of arranging the RH bulk body in the processing chamber, introducing a holding member and a net, putting the upper RH bulk body on the net, and sealing the processing chamber hermetically and carrying out an evaporation diffusion. These techniques have contributed to increasing HcJ without decreasing Br by using only a little Dy.